Geology Reference
In-Depth Information
Box 10.3 Mass spectrometry
Because the isotopes of an element have virtually identi-
cal
chemical
properties, routine spectrometric methods of
analysis that discriminate between elements by means of
emitted spectral wavelengths (Box 6.3) cannot distinguish
between an element's isotopes. The one property by which
isotopes differ significantly is
atomic mass
. A mass spec-
trometer is an instrument that separates isotopes on this
basis and measures their relative abundances.
When ionized, the isotopes of an element share the
same ionic charge but differ in ionic mass (reflecting the
different
A
-values that distinguish the isotopes) and there-
fore in mass/charge (
m/q
) ratio too. Mass spectrometric
isotope analysis consists of five stages:
5
Detection
of ion beams by carefully positioned collectors
that register the intensity of each ion beam, reflecting
the abundance of each isotope.
A mass spectrometer (stages 2-5) needs to be evacuated
to high vacuum to avoid the ions in the narrow beam being
scattered by collision with air molecules.
Modern mass spectrometers employ a number of adj-
acent collectors so that all relevant isotope ion beams can be
measured simultaneously to deliver high-precision isotope
ratios. Automated computer control of accelerating voltage,
magnetic field strength and collector position allows a single
mass spectrometer to be reconfigured to analyse a range of
isotope systems.
1
Separation
- before being introduced into the mass spec-
trometer, the element whose isotopic composition is to
be determined needs to be chemically separated from
other elements in the sample that might interfere.
3
2
Ionization
- the separated element is introduced into the
mass spectrometer's ion source chamber as solid, sol-
ution or gas, where it is ionized in one of several ways
(thermal ionization on a hot filament, inductively cou-
pled plasma, electron impact).
3 Electrostatic
acceleration
of the ions released, through
aligned slits to form a narrow ion beam (Figure 10.3.1).
4
Deflection
and
dispersion
of the ion beam in a strong
magnetic field, causing each isotope component to
emerge in a slightly different direction, according to
m/q
ratio (Figure 10.3.1).
Ion path passes
between upper
and lower pole-
pieces of the
electromagnet.
Magnetic sector
(electromagnet)
Higher-mass
ion beam
Flight tube
Lower-mass
ion beam
Ion beam
Accelerator
slits
High vacuum
Ions
Ion beam
collectors
Ion source
Figure 10.3.1
Sketch of a mass spectrometer viewed from
above, showing ion source, flight tube, magnetic sector (with
electromagnet pole pieces above and below the page), disper-
sion of the ion beam according to
m/q
, and ion collectors.
Modern mass spectrometers typically have 5-10 ion collectors
whose positions can be varied under computer control.
3
For example, to prevent measurement of a
87
Sr peak
being biased by the presence of
87
Rb (an example of isobaric
interference).
the gradient (
m
) of the line equals
e
λ
Rb
−1
. The age of
the intrusive complex can be calculated from the meas-
ured gradient of the isochron in Figure 10.4 as
t
(Table 10.1). Equation 10.2 contains
two
unknowns that
can be measured from the isochron plot:
t
, the age we
are seeking to measure, and the
initial Sr isotope ratio
(
87
Sr/
86
Sr)
0
.
In the context of Figure 10.4, Equation 10.2 has the same
form as the equation for a straight line (see Appendix B):
(
)
ln gradient
+
1
t
=
years
4
(10.4)
λ
Rb
ycxm
=+.
(10.3)
Where the isochron line intercepts the
y
-axis (the inter-
cept
c
) defines the
initial Sr isotope ratio
(
87
Sr/
86
Sr)
0
, and
The units in which
t
is expressed here are the inverse of those
used for
λ
Rb
(Table 10.1).
4
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